THE NATURE OF LAPONITE AND ITS AQUEOUS DISPERSIONS

A study has been made of the physical properties and aqueous dispersion characteristics of the synthetic magnesium silicate Laponite XLG. Results obtained from electron microscopy, electron diffraction, ultracentrifuge analysis, and small angle X-ray scattering studies indicate that the primary particles of this material are thin plate-like crystals of reasonably uniform size, having (plate) diameters of ca. 20 nm. In dilute, colloidally stable aqueous dispersions the average thickness of discrete crystals is within the range 2-4 nm. Significant dissolution of this Laponite material occurs in aqueous solutions of pH < 9 with solution concentrations of magnesium exceeding 10-3 mol dm-3 at pH ≤ 7. In the region of pH 7-8 precipitation of colloidal silica results in a marked increase in the cation-exchange capacity (CEC) of Laponite dispersions. CEC values obtained for dispersions having pH values within the range 8-10 indicate that the Laponite particles develop no significant pH-dependent surface charge in aqueous electrolyte (NaCl) solutions. The results of this investigation indicate that with regard to particle morphology and high pH surface charge characteristics Laponite XLG constitutes a reasonable model material for studies of the behavior of natural smectitie clay minerals. It is clear however that the use of Laponite for such studies requires careful attention to dispersion preparation and "purification" methods and to the choice and control of experimental pH ranges. © 1992.